1. Field of the Invention
The present invention relates generally to flight control systems for helicopters. More specifically, the invention relates to a fully automatic or semi-automatic (e.g. flight director) control system for helicopters for controlling the approach of the helicopter and a hover condition without the requirement of sophisticated and expensive low airspeed sensors, Doppler radar or precision position sensors.
2. Description of the Prior Art
The automatic and semi-automatic control of a helicopter from some initial, presumably high airspeed to a hover condition at zero or substantially zero airspeed has successfully been accomplished in the past; see for example the present assignee's U.S. Pat. Nos. 3,521,838 and 3,916,688 to R. S. Buffum et al and J. C. Dendy et al respectively. As explained fully therein, the control of helicopter longitudinal velocity requires accurate measures of helicopter acceleration, velocity and position and sensors of these parameters such as low-speed air data sensors, Doppler radar and microwave landing system receivers, are very complex and costly. The above Buffum et al patent describes a system for precisely hovering a helicopter using accelerometers to maintain tight velocity control on a short term basis. However, long term stability depends upon the verticality errors (long term drift and effects of acceleration) of the vertical reference used to stabilize the accelerometer. The above Dendy et al patent describes an approach to hover control system wherein the acceleration derived velocity term is improved by including an accurate measure of range-to-hover from a precision approach radar or microwave landing system. Since the latter system requires ground based references, its use is limited to specific geographical areas.
The present invention provides performance comparable with the above more sophisticated systems at a fraction of their costs.